1. Field of the Invention
The present invention relates to a cellulose acylate film having suitably adjusted retardation and excellent adhesiveness to a polarizer and capable of being directly stuck to a polarizer, to a method for producing the cellulose acylate film, and to a retardation film using the cellulose acylate film and, further, to a highly reliable polarizing plate and a liquid crystal device using the cellulose acylate film.
2. Description of Related Art
A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizing plates, and image display devices. Films that are more excellent in the point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films.
Of these, cellulose acylate films having suitable moisture permeability can be directly stuck to the most popular polarizers formed of polyvinyl alcohol (PVA)/iodine in on-line operation. Accordingly, especially cellulose acetate is widely employed as a protective film for polarizing plates, and methods for producing it have been variously investigated (see, for example, JP-A-2001-188128 and JP-A-2000-352620).
In the case of applying a cellulose acylate film to optical use, for example, in retardation films, supports for retardation films, protective films for polarizing plates, and liquid crystal display devices, the control of adhesiveness of the cellulose acylate film to other member is an important factor which determines the reliability of display devices. For example, in the case of applying a cellulose acylate film to use in a protective film for a polarizing plate, inadequate adhesiveness to the polarizer might lead to serious reduction in polarizing degree or separation of the polarizer from the protective film under some using conditions. As a result, there results deterioration of the performance of the polarizing plate and, in the case of using it in liquid crystal display devices, there results deterioration of image quality.
On the other hand, with the late-year demand to enlarge a viewing angle of a liquid crystal display device, improvement of compensation performance of retardation has been increasingly required. At the same time, it has also been required to produce retardation films at a low production cost. Control of such retardation has in many cases been achieved by, for example, adjustment of materials, adjustment of film-forming method, or operation of stretching film (see, for example, European Patent No. 0911656, JP-A-5-257014, JP-A-2005-138358, and JP-A-9-216955).
Also, with respect to relation between the film and the polarizer, various investigations have so far been made to improve adhesiveness between them. However, methods in the related art are failing to provide sufficient adhesiveness, and more improvement has been demanded (see, for example, JP-A-9-216955, WO 02/046809, JP-A-2007-279621.
However, it has become apparent that, when a cellulose acylate film with a retardation having been adjusted by the operation of stretching or the like is produced, the resulting film tends to suffer formation of cracks or, when such a film is stuck to a polarizer to prepare a polarizing plate, there results a deteriorated adhesiveness to the polarizer. As a problem with adhesiveness of the polarizing plate, there has been known formation of cracks or peeling upon cutting or punching the polarizing plate. Additionally, this problem has been reported with polarizing plate using non-stretched cellulose acylate film, but such a problem has become non-problematical at the present technical level of the processing. However, when a polarizing plate is used by using a cellulose acylate film with retardation having been adjusted by the operation of stretching or the like, phenomena different from the adhesion problem in the related art have appeared. In some cases, sticking failure occurs when sticking the polarizing plate to a liquid crystal cell via an adhesive or the like. In such cases, in order to re-use the expensive liquid crystal cell, it is necessary to completely delaminate the polarizing plate from the liquid crystal cell. However, there arises the problem that the polarizing plate with insufficient adhesion suffers tearing upon delaminating operation, thus the polarizing plate remaining on the liquid crystal cell.
On the other hand, with polarizing plates using non-stretched cellulose acylate film, it has been reported that the problems such as formation of cracks or delamination in the step of cutting or punching the polarizing plate have been solved by applying a solvent to the film to thereby reduce out-of-plane orientation to a depth of about 10 μm from the surface. However, it has become apparent that, when such a method is applied as it is to a stretched cellulose acylate film, not only the out-of-plane orientation but the in-plane orientation is reduced as well, leading to large change in retardation.